A trunked digital radio communications system may be used to provide point to point radio communications between single users or groups of users. A user may access the trunked digital radio communications system by placing a call with a radio or user terminal and transferring voice or data information to another user terminal or group of user terminals. The information may be transferred between the user terminals on communications channels (working channels). The trunked digital radio communications system may provide multiple working channels to enable multiple users to share the trunked digital radio communications system. Free working channels may be assigned as calls are initiated within the trunked digital radio communications system. When a call is completed, the assigned working channel may be released and subsequently may be assigned to a new call.
In some trunked digital radio communications systems, a control channel may provide the working channel assignment for a particular call. For example, when a call is initiated, the working channel assignment for the call may be transmitted on the control channel. Consequently, user terminals within the trunked digital radio communications system may monitor the control channel.
A user terminal may have a user terminal identifier that identifies the user terminal within the trunked digital radio communications system. Similarly, multiple user terminals may be designated as part of a group with a corresponding group identifier. A call may, therefore, be placed to a particular user terminal or group of user terminals within the trunked digital radio communications system by indicating the proper identifier. For example, the user terminal may include a keypad to enter the user terminal identifier of the particular user terminal to be called. Alternately, the group identifier may be entered, thereby broadcasting the call to the group. A user may then initiate a call within the trunked digital radio communications system by pressing a push-to-talk (PTT) button on the user terminal. The call is then transmitted using the identifier entered by the user.
Some existing trunked digital radio communications systems may allow an on-going call between user terminals to be interrupted by a higher priority call that is transmitted to at least one of user terminals participating in the call. For example, a user terminal may include a priority switch so that the user can select the priority of the call. Subsequently, a call transmitted to a user terminal has the selected priority. Accordingly, when the high priority call is transmitted, the identified user terminal (or group of user terminals) is interrupted with the higher priority call.
When the identified user terminal (or group of user terminals) receives the interruption, the identified user terminal (or group of user terminals) switches or transfers to the higher priority call, wherein the higher priority call is transmitted on a different working channel. The user terminal typically returns to the control channel to receive the working channel assignment for the higher priority call and then transfers to the assigned working channel. However, by the time the user terminal transfers to the control channel, receives the assigned working channel, and transfers to the assigned working channel (such as in TDMA systems that use FDMA control channels), the higher priority call may have ended. Moreover, some control channels may not be synchronized to the assigned working channel, thereby possibly increasing the time to transfer to the higher priority call. In view of the above discussion, there exists a need for improved trunked digital radio communications systems.